The standard hammer-drill chuck has a chuck body that is rotationally fixed but limitedly axially reciprocal on the drill spindle. This body is formed with an axially throughgoing passage into the rear end of which the drive spindle is engaged and into the front of which the shank of a drill bit is engaged. In addition the body is normally formed with three guide bores inclined to the axis and having inner front ends opening into the hole at the front end of the passage and outer rear ends opening at a radially outwardly open groove on the body. Respective jaws slidable in these guide bores have front ends engageable radially with the shank of the drill bit and rear ends formed with radially outwardly projecting teeth exposed in the groove.
The jaws are normally urged radially outward by springs and are displaced radially by an adjustment ring which is provided in the groove and which has a screwthread that mates with the teeth on the jaws. This ring is formed of a pair of semicircular parts secured together by a knurled sleeve. As a rule the radially inner surface of the ring rides directly on the floor of the groove, so that this ring is supported radially in front of and in back of the apertures where the guide bores open into the groove.
The jaws hold the drill bit so it is centered on the spindle axis, and the front end of the drive spindle acts as a hammer on the rear end of the drill bit. The spindle rotates and axially reciprocates slightly, with the rotation being transmitted via the body and jaws to the bit and the reciprocation being effective directly on the bit. Thus the bit normally moves somewhat axially relative to the jaws.
In order to prevent the chuck from loosening during drilling, a particularly vexsome problem in a hammer drill, it is standard to provide a locking sleeve that has teeth axially forwardly engageable with backwardly directed teeth on the adjustment ring. A spring urges this sleeve forward into a position with the teeth meshing. A stop on the chuck body can engage in either of two adjacent axially forwardly open slots in the sleeve. One of these slots is fairly long so that when the stop is engage in it the sleeve can move forward enough to allow the the teeth to mesh, and in this position the stop is blocked angularly by the sides of the long slot so that the sleeve and also the adjustment ring are prevented from rotating on the chuck body.
The other slot is shorter so that when the stop is engaged in it the teeth cannot mesh. This short slot therefore serves to hold the sleeve back out of engagement with the adjustment ring while same is being rotated to clamp or release a bit from between the jaws controlled by the ring. Thus when a bit is being changed the sleeve is pulled back and rotated somewhat to align the stop with the short slot and the sleeve is released so that it locks in the back position. Once the new bit is in place the sleeve is again pulled back and released when in alignment with the long slot to lock the adjustment ring on the chuck body.
In use it is possible for the clamping jaws to work their way into the shank of the drill bit, thereby loosening the connection between this bit and the chuck. In such a situation it is necessary for the user to unlock the adjustment sleeve, retighten the chuck, and relock the sleeve. Obviously this is not convenient, so that instead the loose connection and the sloppy drilling produced thereby is tolerated until drilling is too difficult or impossible. This is a particular problem with hammer drilling where the jaws should rotationally couple the chuck body and drill bit while permitting limited relative axial movement.